compute omega/chunk command

Syntax

Examples

compute1fluidomega/chunkmolchunk

Description

Define a computation that calculates the angular velocity (omega) of
multiple chunks of atoms.

In LAMMPS, chunks are collections of atoms defined by a compute chunk/atom command, which assigns each atom
to a single chunk (or no chunk). The ID for this command is specified
as chunkID. For example, a single chunk could be the atoms in a
molecule or atoms in a spatial bin. See the compute chunk/atom doc page and Section 6.23 for details of how chunks can be
defined and examples of how they can be used to measure properties of
a system.

This compute calculates the 3 components of the angular velocity
vector for each chunk, via the formula L = Iw where L is the angular
momentum vector of the chunk, I is its moment of inertia tensor, and w
is omega = angular velocity of the chunk. The calculation includes
all effects due to atoms passing thru periodic boundaries.

Note that only atoms in the specified group contribute to the
calculation. The compute chunk/atom command
defines its own group; atoms will have a chunk ID = 0 if they are not
in that group, signifying they are not assigned to a chunk, and will
thus also not contribute to this calculation. You can specify the
“all” group for this command if you simply want to include atoms with
non-zero chunk IDs.

Note

The coordinates of an atom contribute to the chunk’s angular
velocity in “unwrapped” form, by using the image flags associated with
each atom. See the dump custom command for a discussion
of “unwrapped” coordinates. See the Atoms section of the
read_data command for a discussion of image flags and
how they are set for each atom. You can reset the image flags
(e.g. to 0) before invoking this compute by using the set image command.

The simplest way to output the results of the compute omega/chunk
calculation to a file is to use the fix ave/time
command, for example:

This compute calculates a global array where the number of rows = the
number of chunks Nchunk as calculated by the specified compute chunk/atom command. The number of columns =
3 for the 3 xyz components of the angular velocity for each chunk.
These values can be accessed by any command that uses global array
values from a compute as input. See Section 6.15 for an overview of LAMMPS output
options.

The array values are “intensive”. The array values will be in
velocity/distance units.